Any history of depression can be construed as mental illness. The stats on depression in this country are staggering.

Who will define mental illness as the basis for not owning firearms?

This is exactly what the gun-ban crowd would like to do. Does anyone have any idea how many tens of millions of people have taken anti-depressants over the last few decades? If you've taken them, that means your doctor has deterined that (at least at the time) you suffered from some degree of mental illness. This WILL be used as a way of denying people the right to keep and bear firearms. If they have no moral qualms about dancing over the still-warm bodies of children victimized by a nut, why would anyone think that they'd have moral qualms about disarming tens of millions of people using some other means (that they can claim is "only reasonable")?